The IEEE 1394 standard, "P1394 Standard For A High Performance Serial Bus," Draft 8.01v1, Jun. 16, 1995, is an international standard for implementing an inexpensive high-speed serial bus architecture which supports both asynchronous and isochronous format data transfers. The IEEE 1394 standard provides a high-speed serial bus for interconnecting digital devices thereby providing a universal I/O connection. The IEEE 1394 standard defines a digital interface for the applications thereby eliminating the need for an application to convert digital data to analog data before it is transmitted across the bus. Correspondingly, a receiving application will receive digital data from the bus, not analog data, and will therefore not be required to convert analog data to digital data. An `application` as used herein will refer to either an application or a device driver.
The cable specified by the IEEE 1394 standard is very thin in size compared to many other cables, such as conventional co-axial cables, used to connect such devices. Devices can be added and removed from an IEEE 1394 bus while the bus is active. If a device is so added or removed the bus will then automatically reconfigure itself for transmitting data between the then existing nodes. A node is considered a logical entity with a unique address on the bus structure. Each node provides an identification ROM, a standardized set of control registers and its own address space.
A standard IEEE 1394 cable is illustrated in FIG. 1. An IEEE 1394 network using the standard IEEE 1394 cable 10 is a differential, copper wire network, which includes two differential pairs of wires 12 and 14, carrying the differential signals TPA and TPB, respectively. As shown in FIG. 1, the pairs of wires 12 and 14 are twisted together within the cable 10. The signals TPA and TPB are both low voltage, low current, bidirectional differential signals used to carry data bits or arbitration signals. The signals TPA and TPB have a maximum specified amplitude of 265 mVolts. The twisted pairs of wires 12 and 14 have a relatively high impedance, specified at 110 ohms, such that minimal power is needed to drive an adequate signal across the wires 12 and 14. The standard IEEE 1394 cable 10 also includes a pair of power signals VG and VP, carried on the wires 16 and 18, respectively. The wires 16 and 18 are also twisted together within the cable 10. The pair of power signals VP and VG provide the current needed by the physical layer of the serial bus to repeat signals. The wires 16 and 18 have a relatively low impedance and are specified to have a maximum power level of 60 watts.
The IEEE 1394 cable environment is a network of nodes connected by point-to-point links, including a port on each node's physical connection and the cable between them. The physical topology for the cable environment of an IEEE 1394 serial bus is a non-cyclic network of multiple ports, with finite branches. The primary restriction on the cable environment is that nodes must be connected together without forming any closed loops.
The IEEE 1394 cable connects ports together on different nodes. Each port includes terminators, transceivers and simple logic. A node can have multiple ports at its physical connection. The cable and ports act as bus repeaters between the nodes to simulate a single logical bus. Because each node must continuously repeat bus signals, the separate power VP wire 18 and ground VG wire 16, within the cable 10, enable the physical layer of each node to remain operational even when the local power at the node is turned off. The pair of power wires 16 and 18 can even be used to power an entire node if it has modest power requirements. The signal VG carried on the wire 16 is a grounded signal. The signal VP carried on the wire 18 is powered from local power of the active devices on the IEEE 1394 serial bus. Accordingly, at least one of the active devices must be powered by local power. Together, the signals VG and VP form a power signal which is used by the nodes.
The cable physical connection at each node includes one or more ports, arbitration logic, a resynchronizer and an encoder. Each of the ports provide the cable media interface into which the cable connector is connected. The standard IEEE 1394 cable connectors, used at both ends of the IEEE 1394 cable 10 provide six electrical contacts plus a shield. The six electrical contacts represent two contacts for each of the differential signals TPA and TPB, and a single contact each for the power signal VP and the ground signal VG. The arbitration logic provides access to the bus for the node. The resynchronizer takes received data-strobe encoded data bits and generates data bits synchronized to a local clock for use by the applications within the node. The encoder takes either data being transmitted by the node or data received by the resynchronizer, which is addressed to another node, and encodes it in data-strobe format for transmission across the IEEE 1394 serial bus. Using these components, the cable physical connection translates the physical point-to-point topology of the cable environment into a virtual broadcast bus, which is expected by higher layers of the system. This is accomplished by taking all data received on one port of the physical connection, resynchronizing the data to a local clock and repeating the data out of all of the other ports from the physical connection.
A maximum cable length of 4.5 meters is specified for an IEEE 1394 cable. The limitations of an IEEE 1394 serial bus are set by the timing requirement of the arbitration protocol for a fixed round-trip time for transmitted signals. The default timing is set after at most two bus resets, and it is adequate for 32 cable hops, each of 4.5 meters, for a total of 144 meters. This maximum cable length is not practical in some environments in which the distance between active devices is greater than 4.5 meters.
A lack of existing IEEE 1394 repeaters means that IEEE 1394 serial busses must be constructed only in environments which lend themselves to the placement of devices within 4.5 meters of each other. In some environments, devices must, by necessity, be separated by more than 4.5 meters. Without an active repeater or longer cables, an IEEE 1394 serial bus is not practical for such configurations.
The IEEE 1394 cable was designed to comply with the Federal Communications Commission (FCC) regulations for Class B consumer electronics devices. However, the standard IEEE 1394 cable does not comply with other federal regulations, set by the Federal Aviation Association (FAA) for equipment which is used on commercial aircraft. The FAA has strict requirements relating to adequate shielding of electromagnetic interference (EMI) radiation and flammability of the cable. The PVC jacket specified for use on an IEEE 1394 standard cable is highly flammable and produces toxic gasses when burned. The IEEE 1394 standard cable also emits a greater amount of EMI radiation than is allowed under the FAA requirements. For these reasons, a standard IEEE 1394 cable cannot be used on commercial aircraft.
What is needed is a cable which is suitable for use between IEEE 1394 devices and which also complies with the FAA regulations for use on commercial aircraft. What is further needed is an apparatus which can be used as an active repeater between IEEE 1394 cables. What is still further needed is an apparatus which can be used as an active repeater between IEEE 1394 cables and which is suitable for use on commercial aircraft.